If the gas turbine unit of the plant cannot rapidly resume normal operating conditions, for example by re-synchronizing of the generator of a power turbine, the very large energy contents of the PFBC plant in the hot bed, in the surrounding pressure vessel and in unburnt fuel in the bed entail special and difficult problems to be solved. When a rapid reconnection of the generator of a power turbine to its power network in not possible, the energy contents in the bed and in the unburnt fuel have to be removed. When shutting off the turbine, the gas flow through the bed vessel and the turbine is reduced to such a level as is determined by an unavoidable leakage flow in the valve in the hot gas conduit between the bed vessel and the turbine. The air supply to the bed becomes insufficient for fluidization of the bed and for complete combustion of fuel present in the bed. This means that the bed collapses with the ensuing risk of the bed material sintering together and the formation of carbon monoxide (CO), which entails a risk of explosion and also means that the leakage flow contains energy-rich combustible gas. Combustion of this gas downstream of the cut-off valve may cause an impermissibly high gas temperature for the gas turbine and the energy contents may give an impermissibly high speed of turbines in the plant.
It has been proposed to blow off the hot combustion gases from the bed vessel of the plant to the atmosphere. The gases have a temperature of 800.degree.-900.degree. C. and are mixed with about 200 ppm dust. It is dufficult--not to say impossible--to cause a valve operating at such a high temperature and in such a severe environment to seal. In addition, it is very difficult to satisfactorily clean the very large flow of gas at such a high temperature. A suitable valve would be expensive and its working life would be short. One way of reducing the inconvenience of leakage in a valve for blowing off combustion gases from the bed vessel is to blow off, at the same time, the compressed combustion air in a pressure vessel surrounding the bed vessel in the manner disclosed in U.S. Pat. No. 4,498,285 to Kreij. The combustion gases are then mixed with air and cooled so that a valve in the pressure vessel wall is not subjected to gases of such extremely high temperatures. A disadvantage is that compressed combustion air is to a large extent consumed for cooling of the combustion gases which leave the bed vessel and do not pass through the bed. This may mean that the fuel in the bed is not completely burnt and that the bed material will not be cooled to the desired extent. The combustible gases may entail a risk of explosion. A high bed temperature after a blow-off may result in the bed material sintering. In a valve in the bed vessel wall, a certain leakage into the bed vessel may be tolerated. However, the method is not completely satisfactory.
U.S. Pat. No. 4,744,212 to Andersson et al discloses a method of overcoming the problems by supplying an inert gas, suitably nitrogen, to the bed vessel when the turbine is being shut off. This interrupts the supply of oxygen to the bed and hence the combustion, thus preventing the formation of carbon monoxide and eliminating the risk of explosion.